Polyethylene pipe has a lot of benefits: It's lightweight, it's easy to handle, it's non-corrosive and it isn't going to snap. And if it's installed correctly, you aren't going to find yourself with leaking joints.
One thing that polyethylene has that other types of pipe don't have is ductility.
You can push poly pipe into shape
In this situation, 'ductility' refers to the flexibility of the pipe: Its ability to be adjusted without fracturing. In engineering, 'ductility' has a specific definition: The ability for a material to undergo deformation before it ruptures or breaks.
The ability for polyethylene pipe to be pushed into shape is what makes it such a useful medium in situations where you can't change the landscape or the environment significantly. For example, you might have a trench that has a slight bend in it: Polyethylene pipe will flex enough to fit into the curve.
But that doesn't mean you can just bend HDPE
The critical thing to remember is that poly pipe is still strong pipe. Its ductility doesn't mean that you are able to grab a piece of pipe and just bend it into whatever shape you want! But what it gives you is some flexibility in installation.
A great example is a burst steel water main, where you need to dig up a road and replace a 12-metre section. If the section isn't quite straight then poly is more suitable - quite apart from the other reasons why you might use it, such as its non-corrosive nature.
This is why it's so critical to put in the right number of supports in. The advantage you get with the flexibility of the pipe---being able to push it a bit this way or that way---also means that you have to be more mindful about how well you support it.
A question that we get a lot is: How much can you bend poly pipe?
In other words: How flexible is it before you actually have to put a bend into it?
The answer really depends on your use case. But here are two situations that might help you to see the pipe's flexibility.
Imagine that you're running a trench in order to put in a 10 km-long pipeline. If you've got a few dog-legs along the way, then if you have plenty of room, you can slowly curve the pipe in the direction of the change in direction.
Or you might be using poly pipe to transport mine tailings. In many tailings situations, the pipe needs to be moved around. If you're using polyethylene pipe, then you can hook it up to the back of a tractor and move it into the right position: It's both lightweight and flexible. You can't do that with steel.
##Warning: Poly pipe is not a hose!
If you require something that is highly flexible, you really need a hose and not a pipeline.
Polyethylene is flexible over long distances, but not over small distances. If you have a small distance and need the pipe to change direction, that's when you'd want to specify a bend in the pipe.
The flexibility of polyethylene allows you to accommodate site conditions that are uneven. You have to be careful, when putting poly pipe into a curved environment, that you don't inadvertently create conditions that will kink the pipe. Kinks introduce local stress that will damage the pipeline.
Remember, too, that the flexibility is great but if it expands or contracts under whatever conditions exist along your pipeline, then you might find it to be a detraction. Always consider the requirements of your pipeline, all of its temperature extremes, and its environment, before determining what to put in.
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